Timing

Video: Precision Time Protocol (PTP) and Packet Timestamping in Linux

Posted on by Russell Trafford-Jones

SMPTE’s ST-2110 standard can work on hardware or software, but its reliance on PTP, the Precision Time Protocol, makes full software support tricky. Why is this? Is this not just a question of more precise coding practices or changing programming language? There are times when PTP support for ST 2110 isn’t strictly needed and, indeed, the IPMX project is currently working on relaxing PTP requirements so that ST 2110 can be used in the ProAV market without ubiquitous PTP. But when you do need it on software deployed on a server, what are your options and what are the challenges?

This talk by Antoine Ténart looks at the pros and cons of using software vs hardware to create timestamps. First, however, Antoine looks at how PTP works. We’ve covered this before in a Cisco talk but Antoine points out that there are two methods that PTP can work, 1-step and 2-step. PTP synchronisation works by sending 2 messages from the grandmaster clock to the clock wanting to synchronise. There are also two messages sent back to the grandmaster. Keeping close track of when each of these messages was sent and received, and assuming the network delay is the same in both directions, you can work out how long it’s currently taking for timing messages to get to you. Once you’ve cracked the secret of how long messages get to you, you can accurately sync your clock to messages from the grandmaster which say ‘the time is currently …”

 

 

Without this exchange of messages, there’s no way to accurately synchronise your time with the PTP grandmaster within nanoseconds and you’d be left with NTP as your best option which can only keep accuracy within a few milliseconds. Some logs, transactions and media need much better accuracy than milliseconds. So with PTP relying on accurate timestamps, it’s important to find the best way to accurately stamp each message with its origin time.

Without hardware support, when the grandmaster sends its first message saying “This is the time”, a second message needs to be sent immediately afterwards saying “By the way, that last message actually left at a slightly different time:…” which is called the Follow-up Packet. Within broadcast, most equipment has hardware support and so can update the packet as it leaves the grandmaster with the actual time. When you can avoid the follow-up packet, this is known as a 1-step process.

As we covered in this the second talk from Cisco there is more than one type of clock: grandmaster, boundary and transparent. Antoine takes a moment to show how the boundary clock fits between end-devices and the grandmaster. For a deeper dive into PTP and its application to broadcast, watch Arista’s Gerard Philips in this IET Media talk.

Source: Antoine Ténart

Antoine tackles both software and hardware timestamping next. Software, he shows is done in the application or the kernel using the system clock. The errors/deltas involved can be big with a long time passing before transmission. Not being certain when timestamps will occur leads to jitter in the timing signal.

Hardware insertion can be done in the Ethernet layer, in PHY or by a dedicated controller like the Mellanox X5 cards. Errors and deltas are small since the timestamp is inserted close to the actual transmission. In fact, the only delta is it crossing the PHY layer.

The video ends with Antoine discussing offloading, specific calls in the kernel such as SO_TIMESTAMPING and SO_TIMESTAMPTING_TX_HARDWARE as well as introducing us to some tools such as ptp4l, which is a PTP client and ptp2sys which updates the system clock to the ptp time.

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Speaker

Antoine Ténart Antoine Ténart
Senior Software Engineer, Red Hat
Former Linux Kernel Engineer, Bootlin

Video: ST-2110 – Measuring and Testing the Data, Control and Timing Planes

Posted on by Russell Trafford-Jones

An informal chat touching on the newest work around SMPTE ST-2110 standards and related specifications in today’s video. The industry’s leading projects are now tracking the best practices in IT as much as the latest technology in IP because simply getting video working over the network isn’t enough. Broadcasters demand solutions which are secure from the ground up, easy to deploy and have nuanced options for deployment.

Andy Rayner from Nevion talks to Prin Boon from Phabrix to understand the latest trends. Between then, Andy and Prin account for a lot of activity in standards work within standards and industry bodies such as SMPTE, VSF and JT-NM to name a but a few, so whom better to hear from regarding the latest thinking and ongoing work.

Andy starts by outlining the context of SMPTE’s ST-2110 suite of standards which covers not only the standards within 2110, but also the NMOS specifications from AMWA as well as the timing standards (SMPTE 2059 and IEEE 1588). Prin and Andy both agree that the initial benefit of moving to IT networking was benefiting from the massive network switches which now delivering much higher switching density than SDI ever could or would, now the work of 2110 projects is also tracking IT, rather than simply IP. By benefiting from the best practices of the IT industry as a whole, the broadcast industry is getting a much better product. Andy makes the point that broadcast-uses have very much pushed fabric manufacturers to implement PTP and other network technologies in a much more mature and scalable way than was imagined before.

Link to video

The focus of conversation now moves to the data, control and timing plane. The data plane contains the media essences and all of the ST 21110 standards. Control is about the AMWA/NMOS specs such as the IS-0X specs as well as the security-focused BCP-003 and JT-NM TR-1001. Timing is about PTP and associated guidelines.

Prin explains that in-service test and measurement is there to give a feeling for the health of a system; how close to the edge is the system? This is about early alerting of engineering specialists and then enable deep faultfinding with hand-held 2110 analysers. Phabrix, owned by Leader, are one of a number of companies who are creating monitoring and measurement tools. In doing this Willem Vermost observed that little of the vendor data was aligned so couldn’t be compared. This has directly led to work between many vendors and broadcasters to standardise the reported measurement data in terms of how it’s measured and how it is named and is being standardised under 2110-25. This will cover latency, video timing, margin and RTP offset.

More new work discussed by the duo includes the recommended practice, RP 2059-15 which is related to the the ST 2059 standards which apply PTP to media streams. As PTP, also known as IEEE 1588 has been updated to version 2.1 as part of the 2019 update, this RP creates a unified framework to expose PTP data in a structured manner and relies on RFC 8575 which, itself, relies on the YANG data modeling language.

We also hear about work to ensure that NMOS can fully deal with SMPTE 2022-7 flows in all the cases where a receiver is expecting a single or dual feed. IS-08 corner cases have been addressed and an all-encompassing model to develop against has been created as a reference.

Pleasingly, as this video was released in December, we are treated to a live performance of a festive song on piano and trombone. Whilst this doesn’t progress the 2110 narrative, it is welcomed as a great excuse to have a mine pie.

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Speakers

Andy Rayner Andy Rayner
Chief Technologist,
Nevion
Prinyar Boon Prinyar Boon
Product Manager,
PHABRIX

Video: PTP in WAN Applications & Update on PTP v2.1

Posted on by Russell Trafford-Jones

PTP is evolving as is our ability to use it over WANs. This video explains what’s new in PTP’s second revision and the evolving techniques of using PTP over a wide area network such as the internet. As PTP was built assuming the use of LANs, the longer and more unpredictable latency of WANs throws off the timing calculations, so what can be done to compensate?

In this video from RAVENNA, Andreas Hildebrand from ALC Networx takes us through PTP 2.1, the 2019 revision of PTP following on from PTP 2.0 in 2008 and from the original 1.0 in 2002. Famously, 2.0 and 1.0 were not compatible with each other which has caused problems with some hardware implementations of DANTE which were first written when 1.0 was the only edition. Importantly, Andreas highlights, version 2.1 is backwards compatible with version 2.0. If you’re looking for a PTP primer before digging in, have a look at this intro video from Daniel Boldt, Meinberg

Andreas explains the use of PTP profiles within both AES67 and SMTPTE 2110 which standardise some of the parameters for using PTP such as message frequency. Whilst they do have different defaults, there is an overlap between the two allowing for use of AES67 streams withing both an AES67 ecosystem and with a 2110-30 ecosystem. These overlaps are detailed in the joint AES and SMPTE document, AES-R16-2016.

“What’s new in PTP v2.1?” asks Andreas. Apart from clearer language, accuracy, flexibility and robustness have been enhanced.

Flexibility
Flexibility comes from the ability to mixed multicast and unicast messages. Announce and sync messages are still sent in multicast, but queries like delayresponse & delayrequest can now be sent unicast which provides better scalability as, for many scenarios, the reply never needs to go back to any other computers. Another example of flexibility is modular transparent clocks i.e. ones in SFPs. Another flexibility improvement is that it’s now possible to isolate profiles without using different PTP domain numbers. This is by adding a Profile ID called ‘SdoId’.

Robustness and Security
PTP now allows inter-domain interactions effectively allowing multiple GMs to vote onto a single ‘multi-domain clock’. This becomes a very robust clock as it’s created from the insight of three grandmasters. PTP v2.1 adds source integrity checking to allow identification of false, injected, messages. A long-needed improvement as security, even of a LAN, can’t be assumed nowadays.

Performance and Accuracy
Stats reporting has been added to PTP v2.1 allowing monitoring of the average, minimum, max and standard deviation of a number of metrics from the leader clock, delay metrics and message reception counters. Accuracy has been boosted to sub-nanosecond by the CERN-related White Rabbit Project which is an overall benefit to PTP even if sub-nanosecond timing isn’t needed.

Source: ALC NetworX

The second part of the video features Meinberg’s Daniel Boldt who discusses how to transmit PTP over the WAN. This is more challenging than a WAN because it’s more likely to be affected by queueing delays – particularly if the WAN in question is the internet. Queueing delays happen for a number of reasons but it all boils down to the fact the switches and routers often have to hold packets in a buffer, something that tends to happen more when there is more load. As such, this means that the delay is variable leading to varying jitter on the measurements.

Another problem often encountered is path changes where a switch happens in the network to divert the signal to a different path. Whilst this is a great way to route around problems, it does mean a sudden change in path length and therefore propagation delay. A conventional ping time may change from 100ms to 250ms in a second. This could have a big impact on the accuracy of a PTP timing signal if undetected.

Finally, the PTP timing algorithm assumes that it takes just as long, and no longer, to get the timing information from A to B as it does to get the follow-up reply from B to A. When one direction takes longer than the other, for instance when one direction is forced through a 100Mbps link rather than 1000Mbps, the PTP timing will have a constant timing error.

Source: Meinberg

Daniel explains that these issues can be mitigated by more thorough processing of the incoming packets. For instance, having a high-quality oscillator which can maintain an accurate frequency for a long time without external input is helpful. Having a local GM on GPS is another good way to avoid problems, in the cases when this is practical, where the WAN PTP becomes an ‘aide’ to timing rather than the authority. Finally, the ‘lucky packets’ technique is demonstrated.

Daniel explains that if you look at the delay of each packet incoming over, say, a two-second period, you can collect all the packets that, based on the timestamp, were lucky enough not to be delayed a lot. By discarding those very-delayed packets, the accuracy of the PTP signal becomes much higher and jitter can reduce, as we see from two case studies, by an order of magnitude.

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Speakers

Andreas Hildebrand Andreas Hildebrand
RAVENNA Evangelist,
ALC NetworX
Daniel Boldt Daniel Boldt
Head of Software Development,
Meinberg

Video: Timing Requirements in Broadcast Applications

Posted on by Russell Trafford-Jones

How does timing for AES67 and SMPTE ST 2110-30 work? All is revealed in this short video by Andrea Hildebrand who explains why we need PTP timing and how we relate the absolute time to the signals themselves.

In a network for audio streams, Andreas starts, we want all the streams to run on their native sample rate, use the same clock, but also want to have the possibility of multiple concurrent streams using different sample rates. Also, it’s important to have a deterministic end-to-end latency and that, when streams arrive, they should be suitably aligned. We achieve all of this by distributing time around the system. Audio has very high accuracy requirements of down to within 10 microseconds for typical 48KHz broadcast signals, but AES11 requires within 1 microsecond which is why the Precision Time Protocol, PTP is used which is defined by the standard IEEE 1588. For more information on PTP, check out our PTP back library

End devices run their own local clocks, synchronised to the PTP on the network. In charge of it all, there is a grandmaster locked to GPS which can then distribute to other secondary clocks which feed the end devices. The end device can generate a media clock from the PTP and by using PTP, different facilities can be kept in time with each other. All media is then timestamped with the time when they were generated. For advice on architecting PTP, have a listen to this talk from Arista’s Gerard Phillips.

RTP is used to carry professional media streams like AES. RTP builds on top of UDP to add the critical timing information we need. Namely, the timestamp but also the sequence number. Andreas looks at the structure of the RTP packet header to see where the timestamp and identifiers go. To follow up on the IT basics underpinning AES67 and SMPTE ST 2110, check out Ed Calverley’s presentation on the topic.

‘Profiles’ are required to link the time of day to media flows – to give the time some meaning in terms of the expected signal. The AES67 Media Profile does this for AES67 as an annexe in the standard. SMPTE use ST 2059 to define how to use AES67 as well as all the other essences it supports and relate them all back to an originating epoch time in 1970.

The talk finishes by looking at the overlap in timing specs for AES67 and ST 2110-30 (AES67 for 2110). For more information on how AES67 and ST 2110 work (and don’t work) together, watch Andreas’s ‘Deeper dive’ on the topic.

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Speakers

Andreas Hildebrand Andreas Hildebrand
RAVENNA Evangelist
ALC NetworX